With the progress of science and technology and rapid development of economy, both urban and rural areas take on a brand-new look, and people's family living conditions and living standard are significantly improved. Handheld four-stroke machines mainly used as garden tools (for example, earth augers, ice augers, ploughing machines and lawn trimmers etc.) undergo a rapid development. Two-stroke engines available at arbitrary direction are predominately used in the present market of small handheld engines. However, as the environmental awareness rises, requirements for exhaust emission are improved continuously. Therefore, it is an inevitable trend that handheld four-stroke engines will take place of handheld two-stroke engines.
However, a handheld four-stroke engine has a number of shortcomings in use. Firstly, the engine must be equipped with a lubricating system enabling efficient circulation of lubricating oil in order to ensure reliable running of a crankshaft connecting rod piston mechanism and a valve mechanism etc. However, an existing lubricating system is unable to deliver lubricating oil rapidly and unimpededly to working chambers due to its zigzag oil ways, complex structure, cumbersome assembly and difficult production and processing, which reduces the cycle efficiency of lubricating oil to some extent. Secondly, when the engine runs at arbitrary rotation angle, pressure distribution in oil return passage inside the lubricating system is uneven due to poor oil-air separation and poor circulation of the lubricating system, which leads to poor running reliability and large consumption of engine oil, directly affecting the running efficiency of the engine. For these reasons, many manufacturers and persons of insight try to develop and manufacture new engines in order to overcome the above-mentioned defects. Unfortunately, an ideal engine is not manufactured until now.